This invention relates to a coating composition containing an organic resinous polymer, commonly synthetic polymers and two pigments, especially useful for coating active metal parts, which coating imparts increased resistance to corrosion. Specifically, the improved resistance to corrosion is associated with chemical attack accelerated by corrosive ions in salt spray and acid rain conditions. The invention also provides for metal parts coated with the composition of the invention and a method of coating the metal parts with said composition. The terms used herein are further explained hereinafter.
Electrochemically active metals such as magnesium, zinc and aluminum are widely used in aerospace, aviation, marine, defense, and other industries as structural materials. They are highly desirable because of their weight/strength ratios, and/or ease of fabrication.
Magnesium, zinc and aluminum are high in electrochemical activity as measured by their standard half cell potentials and their galvanic potentials. These metals have common "surface" chemistry. These metals generally form a natural protective film of hydroxide, oxide or carbonate in most environments. However, these films dissolve or are attached in acid solutions and corrosion occurs by pitting and general attack. In particular, these natural protective films are more rapidly attacked as the environment increases in acidity and specifically in the presence of certain ions, including chloride and sulfate ions. Often sulfate, sulfide and chloride ions are found together in precipitation.
There is a pressing and serious need to provide coating compositions and coated metal parts of such galvanically active metals to render these parts less prone to corrosion and more resistant to such environmental damage.